T cell activation through the T cell receptor and costimulatory molecules such as CD28 has been demonstrated to influence the susceptibility of T cells to HIV-1 infection and regulate proviral transcription. However, the signaling events initiated by the T cell receptor complex and CD28 that /directly impact HIV-1 transcription have not been explored. In particular, it is unclear how CD28 signaling potentially enhances and inhibits HIV-1 expression. The objective this project is to determine the mechanism by which CD28 regulates HIV-1 transcription, and to examine the interaction between HIV-1 Nef and CD28 signals. We hypothesize that CD28 engagement results in distinct signaling cascades that have very different consequences for HIV-1 expression and that the HIV-1 encoded protein Nef contributes to aberrant T cell signaling and function by interacting with different components of the CD28 signaling cascade. Our preliminary data using chimeric CD28 receptors that include mutations in critical tyrosine residues as well as inhibitors that block specific signal transduction pathways supports such a model. Furthermore, these initial experiments demonstrate phophatidylinositol-3-kinase inhibits H IV-1 transcription by a Tat-dependent mechanism. We are proposing to extend these studies and use cell lines, primary systems and biochemical approaches to 1) further characterize the role of differential signals from CD28 in regulating HIV-1 transcription, 2) characterize cis-elements and transcription factors that mediate induction of LTR activity in response to CD28 signals and 3) determine if Nef directly or indirectly alters CD28 signaling. Understanding the mechanisms by which CD28 regulates HIV-1 transcription will further define pathways associated with this receptor as well as identify putative upstream signal transduction events critical for controlling HIV-1 expression. Furthermore, the ability to manipulate these pathways may provide unique therapeutic targets for controlling virus expression.